The present invention relates generally to amplifier circuits and more specifically to headphone amplifier circuits in portable electronic devices.
In portable electronic devices, such as portable CD players and MP3 players, audio signals are amplified by headphone amplifiers. A headphone amplifier is typically externally biased by two power supplies to accurately represent incoming signals. However, portable electronic devices usually use a single power supply, e.g., a battery, as the only source of power. In a prior art headphone amplifier circuit shown in FIG. 1, headphone amplifiers 101 and 102 have their positive power supply terminals coupled to a positive power supply VDD and their negative power supply terminals coupled to ground, and consequently are biased at mid-rail (VDD/2). Since the output of the amplifiers 101 and 102 are at a higher DC voltage with respect to ground, relatively large capacitors 103 and 104 are required to prevent high currents from flowing through headphones. Such large capacitors make miniaturizing portable electronic devices difficult.
In another prior art headphone amplifier circuit shown in FIG. 2, amplifiers 201 and 202 have their positive power supply terminals coupled to VDD and their negative power supply terminals coupled to −VDD, and are biased at ground (0 volt) potential. Although large DC blocking capacitors are avoided, the amplifier circuit shown in FIG. 2 is a class AB amplifier, and has low power efficiency.
Another prior art approach is to use a class D amplifier, which has better power efficiency than a class AB amplifier, but high EMI (Electromagnetic interference), which can be a problem.
Therefore, it may be desirable to provide a headphone amplifier circuit which may help to conserve power and avoid audio clipping.